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The principle of equivalence deals with one frame of reference. The equivalence is between two interpretations of a measurement: an accelerated frame of reference or the presence of gravity yield the same result, and there is no way to distinguish between them.

My suggesting that we add scales sent us off in the wrong direction, because that permits us to explore the non-uniformities in the acceleration. The premise of the original discussion was that all the two observers could see was each other. From such a measurement, neither could distinguish whether they were accelerated or experiencing gravity.

By doing a series of experiments, the changing accelerations (or the equivalent changing gravitational fields) suggest the presence of a large mass.

Somehow, I think we need to get into the mathematics of general relativity to sort this out. So I'm signing off because I am about to fall over the edge of my knowlege :)

Fred Bortz -- Science and technology books for young readers (www.fredbortz.com) and Science book reviews (www.scienceshelf.com)